1868.] 



On the Inequalities of Terrestrial Magnetism, 



163 



annual inequality to be an excess of 0'*227 of easterly declination in 

 the months October to March over its value in the months April to 

 September. 



III. " On the Diurnal and Annual Inequalities of Terrestrial Magne- 

 tism, as deduced from observations made at the Royal Observa- 

 tory, Greenwich, from 1858 to 1863 ; being a continuation of a 

 communication on the Diurnal Inequalities from 1841 to 1857, 

 printed in the Philosophical Transactions, 1863. With a Note 

 on the Luno-diurnal and other Lunar Inequalities, as deduced 

 from observations extending from 1848 to 1863." By George 

 Biddell Airy, Astronomer Royal. Received July 27, 1868. 



(Abstract.) 



The author states that the instruments employed are precisely the same 

 which were used in the second part of the former investigation, from 1848 

 to 1857, mounted in the same place, and treated in the same manner. In 

 describing the treatment of the photographic curves, he first gives the 

 number of days which have been omitted in different years ; because the 

 character of the observations or curves was too disturbed to permit the 

 usual treatment of the observations, or the drawing by hand of a pencil 

 curve that would fairly represent the general course of the curve. 



The greatest numbers of omitted days occur in the years 1846, 1847, 

 1848; 1851, 1852, 1853, 1854; 185*9, I860. As the estimate of the 

 amount of irregularity has been made throughout by the same person, he 

 considers that these years may be accepted as those in which the disturb- 

 ances were the greatest. If they point to any cycle at all, it is one of 6 or 

 6J years. These days being omitted, the ordinates of the pencilled curves 

 on the other days were used as basis of all the following investigations. 

 For the solar inequalities, they were treated by collecting the measures for 

 every complete solar day, or for every solar hour bearing the same ordinal 

 number, according as the annual or diurnal inequalities were the subject 

 of inquiry ; but in all cases these quantities were next grouped by months, 

 and the monthly means were taken . 



In the further treatment, the means of the monthly means of every 

 complete day for all the months of the same name in the different years 

 were taken and corrected for secular change; the corrected numbers do 

 not appear to indicate any sensible annual equation. Then the means of 

 the monthly means of every solar hour for all the months of the same name 

 in the different years were taken, giving the diurnal inequalities on the 

 mean of years for the twelve separate months ; and these present, for the 

 declination (north to west) and horizontal force, for the period 1858 to 

 1863, sensibly the same differences between the summer months and the 

 winter months as those for the period 1848 to 1857. For the vertical-force 



